1. Field of the Invention
The present invention relates to a zero current detecting circuit and related synchronous switching power converter, and more particularly, to a zero current detecting circuit and related synchronous switching power converter for improving power converting efficiency.
2. Description of the Prior Art
A switching power converter is an electronic device performing power conversion through switching switches, and is widely used in electronic products with a power supply. For a synchronous switching power converter based on inductors, when the load is light, the converting efficiency can be increased through a discontinuous mode. For example, FIG. 1 is a schematic diagram of a synchronous switching power converter 10. The power converter 10 operates in the discontinuous mode, so that the current on the inductor 104 will not be negative. When the up-bridge transistor 101 is turned on, the inductor 104 is charged. When the down-bridge transistor 102 is turned on, the inductor 104 is discharged, so that the current on the inductor 104 is gradually decreased down to zero. When the current on the inductor 104 is decreased to zero, the down-bridge transistor 102 has to be turned off immediately, so that the current on the inductor 104 will not be negative.
If the down-bridge transistor 102 is not turned off accurately at the moment of the current on the inductor 104 being zero, the converting efficiency of the power converter 10 in the discontinuous mode will be decreased. For example, if the down-bridge transistor 102 is turned off before the moment of the current on the inductor 104 being decreased to be zero, the body diode of the down-bridge transistor 102 will be turned on, which causes the conduction loss and the converting efficiency is decreased. Oppositely, if the down-bridge transistor 102 is turned off after the moment of the current on the inductor 104 being decreased to be zero, the voltage at the node SW will be suddenly increased, which causes switching loss at the down-bridge transistor 102, and the converting efficiency is decreased as well.
Therefore, it is very important to turn off the down-bridge transistor 102 accurately so that the current on the inductor 104 can be decreased to be zero without being negative for the power converter design. In the prior art, the voltage across the resistor RS is measured to determine if the current on the inductor 104 is decreased to be zero. When a comparator 106 of a control circuit 108 measures the voltage on the resistor RS is zero, the output of the comparator 106 changes its state, and thus the control circuit 108 outputs a signal to turn off the down-bridge transistor 102.
Ideally, when the current on the inductor 104 is decreased to be zero, the down-bridge transistor 102 should be turned off to limit the conduction and switching loss of the power converter 100. However, in practice, an offset voltage exists in the comparator 106, and therefore the moment of the voltage on the resistor RS being zero cannot be accurately determined. Consequently, the down-bridge transistor 102 cannot be turned off accurately at the moment of the current on the inductor 104 being zero, and the conduction and switching loss of the power converter 10 cannot be effectively reduced.